WASHINGTON — The American Bar Association has said that President George W. Bush is flouting the Constitution and undermining the rule of law by claiming the power to disregard selected provisions of bills he signed.

In a report, a bipartisan 11-member panel of the bar association said that Bush had used so-called signing statements far more than his predecessors, raising constitutional objections to more than 800 provisions in more than 100 laws on the ground that they infringed on his prerogatives.

In the ABA panel report released Sunday, members said that these broad assertions of presidential power amount to a line-item veto and improperly deprive Congress of the opportunity to override the veto.

For example, in signing a statutory ban on torture and other national security laws approved by Congress, Bush reserved the right to disregard them.

The bar association panel said that the use of signing statements in this way was "contrary to the rule of law and our constitutional system of separation of powers." From the dawn of the Republic, it said, presidents have generally understood that, in the words of George Washington, a president "must approve all the parts of a bill, or reject it in toto."

If the president deems a bill unconstitutional, he can veto it, the panel said, but "signing statements should not be a substitute for a presidential veto."

The panel's report adds momentum to a campaign by scholars and members of Congress who want to curtail the use of signing statements as a device to augment presidential power.

At a recent hearing of the Senate Judiciary Committee, the chairman, Arlen Specter, a Republican from Pennsylvania, said that Bush seemed to think he could "cherry-pick the provisions he likes and exclude the ones he doesn't like."

Senator Patrick Leahy of Vermont, the senior Democrat on the committee, said that the signing statements were "a diabolical device" to rewrite laws enacted by Congress.

Presidents have long assumed the prerogative of signing statements, which detail portions of legislation they object to and may possibly not enforce.

Justice Department officials dismiss such criticism as unjustified.

"President Bush's signing statements are indistinguishable from those issued by past presidents," said Michelle Boardman, a deputy assistant attorney general. "He is exercising a legitimate power in a legitimate way."

Michael Greco, the president of the bar association, who created the study panel, said that its report highlighted a "threat to the Constitution and to the rule of law."

The panel said, "Our recommendations are not intended to be, and should not be viewed as, an attack on President Bush." The panel said it was equally concerned about the precedents being set for future chief executives.

The panel acknowledged that earlier presidents, including Andrew Jackson, Ulysses Grant, Theodore Roosevelt and Franklin D. Roosevelt, had occasionally asserted the right to disregard provisions of a law to which they objected. Under Bill Clinton, the Justice Department told the White House that the president could "decline to execute unconstitutional statutes."

But the panel said that Bush had expressed his objections more forcefully, more often and more systematically, "as a strategic weapon" to influence federal agencies and judges.

In his first term, the panel said, Bush raised 505 constitutional objections to new laws. On 82 occasions, he asserted that he alone could supervise, direct and control the operations of the executive branch, under a doctrine known as the "unitary executive."

Whenever Congress directs the president to furnish information, Bush reserves the right to withhold it. When Congress imposes mandates and requirements on the executive branch, the president often says he will read them as advisory or "precatory."

When Congress tries to define foreign policy - for example, on Russia, Syria, North Korea or Sudan - Bush objects. Even if he agrees with the policy, he asserts that the congressional directives "impermissibly interfere with the president's constitutional authority" to conduct foreign affairs.

Whenever Congress prescribes qualifications for presidential appointees, Bush complains it is an intrusion on his power, even if Congress merely requires that the appointee know about the field for which he will be responsible.

The panel urged Congress to pass a law requiring the president to "set forth in full the reasons and legal basis" for any signing statement in which he says he can disregard or decline to enforce a statute. The panel also suggested legislation to provide for judicial review of signing statements.

The ABA panel was headed by Neal R. Sonnett, a criminal defense lawyer in Miami. Members include former Rep. Mickey Edwards, R-Okla.; Bruce E. Fein, a Justice Department official in the Reagan administration; Harold Hongju Koh, the dean of Yale Law School; William S. Sessions, a former director of the FBI; Kathleen M.